The refractive index of any medium other than a vacuum varies with wavelength. Thus the Gaussian and aberrational properties of any refracting optical system are functions of wavelength, i.e. Chromatic aberrations exist. Many modern refracting systems intended for use over an appreciable range of wavelengths are ultimately limited in performance by chromatic effects, both Gaussian and higher order. The history of astronomical telescope design provides a useful example. Sir Isaac Newton invented the reflecting telescope because he considered that it was impossible to correct for the chromatic effects in singlet lenses by combining two to form what we now call an achromatic doublet; in effect he thought that the chromatic effect of all lenses were proportional to their powers, with the same constant of proportionality even for different glasses. Then, in the middle of the eighteenth century, John Dollond and Chester Moore Hall showed that Newton had been wrong and they made achromatic doublets. This resulted in larger and larger telescope doublet objectives being made. However, as objectives became larger and design techniques became subtler it was found that an “achromatic” doublet was not quite free from chromatic aberration and a residual error, known as “secondary spectrum”, appeared.
As mentioned, when different colors of light propagate at different speeds in a medium, the refractive index is wavelength dependent. This phenomenon is known as dispersion. A well-known example is the glass prism that disperses an incident beam of white light into a rainbow of colors. Photographic lenses comprise various dispersive, dielectric glasses. These glasses do not refract all constituent colors of incident light at equal angles, and great efforts may be required to design an overall well-corrected lens that brings all colors together in the same focus. Chromatic aberrations are those departures from perfect imaging that are due to dispersion. Whereas the Seidel aberrations are monochromatic, i.e. they occur also with light of a single color; chromatic aberrations are only noticed with polychromatic light.